I would just tell them that guns are used to hurt people and animals and that they aren't very nice.
We haven't discovered guns yet, and I won't tell him he can't pretend to play with them, I will tell him what they are for and that they aren't nice.

I think letting them play with pretend guns helps to desensitise them a bit. Just like I wouldn't want him to pay with a knife I don't want him thinking guns (even pretend ones) are toys. Itms?